Long-term plasticity: depression and synaptic potentiation
Because homosynaptic long-term depression (LTD) has scarcely been examined in the two anatomical divisions of the perforant path to the DG, different patterns of low-frequency stimulation (LFS) were used to examine the LPP’s and MPP’s susceptibility to express a stable LTD. A baseline response of fEPSP slope (50-70% of maximal fEPSP amplitude) was acquired with paired stimulation (60 ms ISI, 100 µs duration of current pulse) at 0.067 Hz for 20 min. Then, 900 unitary current pulses at 1-3 Hz were delivered at the LPP or the MPP, and the synaptic responses were recorded for 90 min. This was followed by pharmacological identification with L-AP4 or DCG-IV (Macek et al., 1996). In the case of MPP – DG synapse, LFS at 3 Hz was examined in the presence of the inverse agonist AM 251 (5 µM), to determine the dependence of cannabinoid receptor 1 (CB1) during the induction of LTD. For long-term potentiation (LTP) experiments, the baseline conditions and pharmacological identification were identical to those in the LTD experiments, except that the baseline response of the fEPSP slope was configured at 25-35% of its maximal amplitude. Then, a theta-burst stimulation (TBS) protocol was delivered to the LPP or the MPP, and synaptic responses were recorded for 90 minutes. The TBS protocol (based on Larson and Munkácsy, 2015) consisted of 3 episodes repeated at 10 s, each with 10 bursts at 5 Hz and 5 current pulses at 100 Hz (see Supplementary Figure 2). The decay of post-tetanic potentiation (PTP) induced by TBS was expressed as 𝜏 value, obtained from adjusting the best fit of individual fEPSP slope values with a nonlinear regression function of one phase decay. Heatmaps were constructed to depict the magnitude of LTP, and cumulative probability charts were constructed using post-TBS fEPSP slope values from minute 11 to 90.